1. Field of the Invention
This invention relates to an image display method and apparatus, an image processing method and apparatus for receiving, reproducing and displaying image information.
2. Description of the Prior Art
Still-picture transmission apparatus utilizing a high-definition television signal have recently been developed. In comparison with the signal used in the present NTSC broadcast television system, the aforementioned high-definition television signal (referred to as an HDTV signal) possesses more than five times as much image information and can provide a fine image with life-like appearance presence since the screen in the HDTV system is wider.
However, since the HDTV signal possesses a great amount of information, even in the case of still pictures, the signal is compressed (coded) by a compressive coding method owing to limitations upon the frequency of broadcast radio waves and communication capacity. Such coding makes it possible to transmit a large quantity of image signals even in a narrow transmission band (or at a low transmission rate). Specifically, in a case where a viewer observes a single still picture for several seconds to several tens of seconds, it has been contemplated to execute transmission at a bit rate of 2 Mbps.
However, the image data transmitted by such an image transmission apparatus represents a still picture, not a moving one. Such an apparatus is incapable of transmitting an image signal which requires the far greater amount of image data involved in a moving picture.
FIG. 5 illustrates the construction of an image receiving/playback apparatus such as a conventional still-picture receiving/playback apparatus, television conference system or television telephone, etc. As shown in FIG. 5, image information which enters from a satellite broadcast line or communication line is converted into a digital image signal by a receiver 501, and the digital signal is sequentially decoded by a coded image decoder 502 to be expanded and restored to the original image data. When one frame of the image data thus decoded is written in a frame memory on a side not currently being used in presenting a display, the content of the frame memory in which this image data has just been written is read out in response to an instruction from the transmitting signal. The image information thus read out of the frame memory designated on the display side is converted into an analog signal by a D/A converter 506, and the analog signal is outputted to a monitor 507, with a synchronizing signal affixed thereto, to be displayed on the screen of the monitor.
In general, when image information received and successively decoded is written in a frame memory on the display side, the following problems occur:
(1) Contention develops among the read timings of the display system.
(2) A difficulty arises when it is attempted to avoid contention between writing and reading in the frame memory by using a multi-port DRAM or the like for the frame memory. Specifically, if the arrangement is such that received image data is newly written in a frame memory whose stored data is being displayed, the display gradually changes from one edge thereof as the writing operation proceeds. The result is a temporarily unattractive picture.
Accordingly, an expedient commonly adopted is to provide at least two frame memories, one for storing image information currently being displayed and the other for storing image information received and decoded. After a complete image has been played back and written in the frame memory on the write side, the frame memory in which the image has been written is switched over to the display side to display its data. Another expedient is to switch the frame memory within the vertical retrace interval, thereby preventing unattractive horizontal lines from appearing momentarily on the screen.
However, the still-picture playback apparatus exemplified by the prior described above plays back only still pictures and is incapable of playing back images as moving pictures.